Electronic circuit and radio communications system

ABSTRACT

The electronic circuit includes: a micro controller unit; a timer operable to measure a standby time of the micro controller unit; a buffer circuit connected with an output-signal terminal of the timer; a logic circuit controlled by an output signal of the timer and an output signal of the micro controller unit; and a power-on switch controlled by the logic circuit. The buffer circuit has no protection diode connected with an input terminal thereof on a power-source side, and connected to a common power source shared with the micro controller unit. The switch is one for controlling the power source of the micro controller unit and buffer circuit. The micro controller unit is made to transition to an active state to either a standby state or a power-source cutoff state during the standby time.

CLAIM OF PRIORITY

The Present application claims priority from Japanese application JP2008-282808 filed on Nov. 4, 2008, the content of which is herebyincorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to a radio communication system and anelectronic circuit used therefor. Particularly, it relates to SensorNet, which is a kind of radio communication system, a sensor node usedas a radio communication device for Sensor Net, and a control circuitused for the sensor node.

BACKGROUND OF THE INVENTION

In recent years, there has been an increase in research and developmentabout a radio communication system, hereinafter referred to as “SensorNet”, in which a terminal having a sensor and radio or wirelesscommunication function, hereinafter referred to as “sensor node”, isused to capture various information pieces of the real world into aninformation processing device in real time. Sensor nodes are placedeverywhere and set on everything, and information pieces gathered by thesensors are sent to a network. The data so gathered are processed intovarious forms, and fed back to the real world. Such Sensor Net has beenunder consideration about applications to various fields such aslogistics, healthcare, and quality control.

To set a sensor node without the need for selecting where to put it, thenetwork connection through a wireless communication and the downsizingthereof are de rigueur. However, a compact-size sensor node isrestricted in the capacity of its battery. Hence, taking into accountthe operational cost including the replacement of a battery, to reduceits power becomes a challenge. For example, Non-patent Documentpresented by Yamashita, S. et al., “A 15×15 mm, 1 μA, reliablesensor-net module: enabling application-specific nodes,” InformationProcessing in Sensor Networks, 2006. vol., no., pp. 383-390, 19-21 Apr.2006, discloses a technique for performing the power reduction bybringing a micro controller unit, hereinafter referred to as “MCU”, toits standby state, and using a timer placed outside MCU to restore it.

In addition, lots of researches on the reduction in power consumption ofelectronic devices have been made. For instance, Japanese UnexaminedPatent Application Publication No. JP-A-9-128106 discloses a centralprocessing unit (CPU) having three operation modes, i.e. a normal mode,a power-saving mode lower in power consumption in comparison to thenormal mode, and a stop mode in which an action is stopped completely.

SUMMARY OF THE INVENTION

Many of sensor nodes which are terminals designed for Sensor Net aredriven by batteries during use. As Sensor Net uses a number of sensornodes, it is required to cut the cost of maintenance thereof. Therefore,sensor nodes need to have a long battery life. In other words, it isessential to achieve a lower power consumption of sensor nodes. For thispurpose, according to the above Non-patent Document presented byYamashita, S. et al., MCU is made to transition to its standby state,and a timer outside MCU is used to restore it, whereby the powerconsumption is reduced. Conventionally, a relatively low-spec MCU hasbeen used for a sensor node, which can be put in a condition of lowpower consumption when made to transition to the standby state. Thestandby state of MCU refers to a condition where MCU remains stoppingsupply of the synchronizing operation clock to a certain internalcircuit such as CPU by e.g. executing a standby command.

As the development of Sensor Net proceeds, a conventional relativelylow-spec MCU becomes insufficient in performance, and the number ofapplications which need high-performance MCU is increasing. An exampleof such applications is an application software program which handles alarge volume of data. Such application software program requires asensor node to compress data, and therefore it needs MCU of a relativelyhigh performance. Further, in some cases, MCU of a relatively highperformance is needed for encryption of data in terms of security.However, even in the case where a high-performance MCU like this isrequired, there is still the demand for reduction of power consumption.In the case of using a high-performance MCU, even if MCU is made totransition to the standby state as in the past, sometimes the effect ofreduction of power consumption cannot be achieved sufficiently becauseof a large current leakage. In addition, the scaling down ofsemiconductor manufacturing processes is going ahead with each passingyear. For example, in the case of using MCU fabricated by amanufacturing process designed for fine semiconductors, the amount of aleak current in the standby state is becoming unignorable. Therefore,required is a means for reducing power consumption unlike theconventional method by which MCU is made to transition to the standbystate. For example, such means is selectively cutting the power supplyto MCU instead of making it transition to the steady state.

However, cutting the power supply to MCU can cause for example, in acondition such that the level of an external input terminal is unstable,an undesired current leakage toward a node of the power supply systemsubjected to the cutting of power supply through an input-protectioncircuit provided corresponding to the external input terminal.Particularly, the study by the inventor showed that in the case of usinga common timer interrupt signal for disabling the standby and the cut ofpower supply, directly supplying an interrupt signal to an interruptterminal can cause a leak current as described above regardless ofwhether the interrupt signal is High-enable or Low-enable one.Specifically, using a Low-enable timer interrupt signal can causecurrent to leak during a period in which the interrupt signal stays atHigh level under the condition where the power supply is cut. In thecase of using a High-enable timer interrupt signal, supplying theinterrupt signal to an interrupt terminal can also cause current to leakbetween the time the cut of power source is disabled by the timerinterrupt signal of High level, and the time the power supply isstabilized.

It is an object of the invention to provide an electronic circuit whichcan select the stop of power supply instead of the transition of a microcontroller unit to the standby state, and which can prevent theoccurrence of unwanted current leakage in the condition of stopped powersupply.

Further, it is an object of the invention to provide an electroniccircuit in which the condition of standby and the condition of stoppedpower supply can be disabled by a common signal.

The above and other objects of the invention and novel features thereofwill be apparent from the description hereof and the accompanyingdrawings.

Of the invention herein disclosed, the preferred embodiments will beoutlined below briefly.

An electronic circuit in connection with the invention includes: a microcontroller unit; a timer operable to measure a standby time of the microcontroller unit; a buffer circuit connected with a signal outputterminal of the timer; a logic circuit controlled by output signals fromthe timer and micro controller unit; and a switch controlled by thelogic circuit. The buffer circuit has no protection diode connected toan input terminal thereof on the power-source side. The buffer circuitis connected to a common power source shared with the micro controllerunit. The switch is a switch for controlling the power source of themicro controller unit and buffer circuit. The micro controller unit canbe made to transition from an active state thereof to either a standbystate or a power-source cutoff state during the standby time, andrestored from the one of the standby and power-source cutoff states tothe active state according to an output from the buffer.

Now, the effects achieved by the preferred embodiments of the inventionherein disclosed will be described below briefly.

The first effect is it is possible to select the stop of power supplyinstead of making a micro controller unit transition to its standbystate. The second is an unwanted current leakage can be prevented frombeing caused in the condition of stopped power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a sensor net system according to anexample of a radio communication system in connection with theinvention;

FIG. 2 is a block diagram showing a sensor node, which is a comparativeexample to a sensor node in connection with the invention;

FIG. 3 is a block diagram showing an example of the sensor node inconnection with the invention;

FIG. 4 is a block diagram showing the first example of a control circuitincluded in the sensor node as shown in FIG. 3;

FIG. 5 is a timing chart of actions of the sensor node;

FIG. 6 is a table for explaining truth values in connection with thecontrol circuit of FIG. 4;

FIG. 7 is a diagram showing specific examples of a logic circuit and aswitch included in the control circuit of FIG. 4;

FIG. 8 is a diagram showing the transition of state of MCU 200;

FIG. 9 is a graph of characteristic curves showing relations between thelife of a battery of the sensor node and the operation cycle;

FIG. 10 is a diagram showing the second example of the control circuitincluded in the sensor node;

FIG. 11 is a timing chart of the control circuit shown in FIG. 10 atpower-on;

FIG. 12 is a table for explaining truth values in connection with thecontrol circuit of FIG. 10;

FIG. 13 is a diagram showing specific examples of a logic circuit and aswitch included in the control circuit of FIG. 10;

FIG. 14 is a timing chart of the control circuit of FIG. 10 at the timeof transition to its standby state;

FIG. 15 is a timing chart of the control circuit of FIG. 10 at the timeof transition to the power-source cutoff state;

FIG. 16 is a diagram showing the third example of the control circuitincluded in the sensor node;

FIG. 17 is a diagram showing the fourth example of the control circuitincluded in the sensor node;

FIG. 18 is a flowchart showing an example of the action flow of thesensor node; and

FIG. 19 is a diagram showing the fifth example of the control circuitincluded in the sensor node.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Summary of thePreferred Embodiments

The preferred embodiments of the invention herein disclosed will beoutlined first. Here, the reference numerals, characters and signs forreference to the drawings, which are accompanied with paired roundbrackets, only exemplify what the concepts of components and elementsreferred to by the numerals, characters and signs contain.

[1] An electronic circuit in connection with the invention includes: amicro controller unit (200); a timer (203) for measuring a standby timeunder the control of the micro controller unit; a buffer circuit (302)which receives an output signal of the timer; a logic circuit (300)controlled by the output signal (Vtim) of the timer, and an outputsignal (Vcnt) of the micro controller unit; and a power-on switch (301)controlled by the logic circuit in switching, and operable to supply apower source to the micro controller unit and buffer circuit. The buffercircuit does not have a protection diode connected with its inputterminal on a power-source side. The micro controller unit is made totransition from an active state thereof to one of standby andpower-source cutoff states during the standby time, and the microcontroller unit is restored from the standby or power-source cutoffstate to the active state according to an output from the buffer.

[2] In the electronic circuit as described in [1], the micro controllerunit decides to transition to the standby state or the power-sourcecutoff state based on a length of the standby time set on the timer.

[3] In the electronic circuit as described in [1], a signal fordirecting the micro controller unit to be restored to the active stateafter the micro controller unit has transitioned to the standby state,and a signal for directing the micro controller unit to be restored tothe active state after the micro controller unit has transitioned to thepower-source cutoff state are identical signals (Vtim), and output bythe timer.

[4] In the electronic circuit as described in [1], the logic circuitswitches the output signal (Vcnt) of the micro controller unit from Highto Low level, thereby to turn off the power-on switch, and to cut apower source of the micro controller unit and buffer circuit. The logiccircuit turns on the power-on switch to power on the micro controllerunit and buffer circuit in response to changeover of the output signal(Vtim) of the timer from High to Low level.

[5] In the electronic circuit as described in [4], after havingtransitioned from the active state to the standby state, the microcontroller unit is restored to the active state in response tochangeover of the output signal of the timer from High to Low level.

[6] In the electronic circuit as described in [1], the micro controllerunit inputs, through its interrupt terminal, the output signal of thebuffer for restoration from the standby state to the active state.

[7] In the electronic circuit as described in [1], the buffer circuithas a protection diode between the input terminal and a ground terminal.

[8] The electronic circuit as described in [1] further includes anactivation circuit (1001) connected with the logic circuit. Theactivation circuit remains waiting for a predetermined length of timeafter its power-on, and then changes its output. The logic circuit keepsthe power-on switch ON until the activation circuit changes its outputregardless of states of outputs from the micro controller unit andtimer.

[9] The electronic circuit as described in [1] further includes aconnector (1704) for debug, which enables supply of a signal for forcingthe power-on switch to shift into ON state.

[10] An electronic circuit in another aspect of the invention includes:a micro controller unit; a timing-generation sensor (1900) operable tooutput a certain detection result; a buffer circuit which accepts anoutput of the timing-generation sensor; a logic circuit controlled bythe output signal of the timing-generation sensor and an output of themicro controller unit; and a power-on switch controlled by the logiccircuit in switching, and operable to supply a power source to the microcontroller unit and buffer circuit. The buffer circuit has no protectiondiode connected to an input terminal thereof on a power-source side. Themicro controller unit is made to transition from an active state thereofto one of standby and power-source cutoff states by the time that thetiming-generation sensor outputs the certain detection result, andrestored from the one of the standby and power-source cutoff states tothe active state according to an output from the buffer.

[11] An electronic circuit in still another aspect of the inventionincludes: a micro controller unit; a timer operable to measure a standbytime under control of the micro controller unit; a buffer circuitoperable to receive an output signal of the timer; a logic circuitcontrolled by the output signal of the timer and an output signal of themicro controller unit; a first regulator (1602) controlled by the logiccircuit in its activation, and operable to supply a power source to themicro controller unit and buffer circuit; and a second regulator (1601)operable to supply a power source to the timer and logic circuit. Thebuffer circuit has no protection diode connected to an input terminalthereof on a power-source side. The micro controller unit is made totransition from an active state thereof to one of standby andpower-source cutoff states during the standby time, and restored fromthe one of the standby and power-source cutoff states to the activestate according to an output from the buffer.

[12] The electronic circuit as described in [1] further has: a sensor(204); a radio communication unit (201); and a power-on switch (206) forradio communication, operable to control a power source of the radiocommunication unit. The power-on switch for radio communication iscontrolled by the micro controller unit. The radio communication unittransmits data from the sensor under control of the micro controllerunit.

[13] A radio communication system according to the invention has aplurality of radio communication devices (100), and a host device (101)which communicates with and manages the plurality of radio communicationdevices, provided that at least one of the plurality of radiocommunication devices is the electronic circuit described in [12].

[14] The electronic circuit as described in [10] further includes asensor (204); a radio communication unit (201), and a power-on switch(206) for radio communication, operable to control a power source of theradio communication unit. The power-on switch for radio communication iscontrolled by the micro controller unit. The radio communication unittransmits data from the sensor under control of the micro controllerunit.

[15] A radio communication system according to the invention has aplurality of radio communication devices (100), and a host device (101)which communicates with and manages the plurality of radio communicationdevices, provided that at least one of the plurality of radiocommunication devices is the electronic circuit described in [14].

[16] The electronic circuit as described in [11] further includes: asensor (204); a radio communication unit (201); and a power-on switch(206) for radio communication, operable to control a power source of theradio communication unit. The power-on switch for radio communication iscontrolled by the micro controller unit. The radio communication unittransmits data from the sensor under control of the micro controllerunit.

[17] A radio communication system according to the invention has aplurality of radio communication devices (100), and a host device (101)which communicates with and manages the plurality of radio communicationdevices, provided that at least one of the plurality of radiocommunication devices is the electronic circuit described in [16].

[18] A radio communication system according to the invention includes: aradio communication terminal; a base station operable to communicatewith the radio communication terminal by radio; a server operable toprocess data passed from the radio communication terminal through thebase station. In radio communication system, the radio communicationterminal includes: a sensor operable to capture data; a radiocommunication device operable to transmit data captured by the sensor tothe base station; a micro controller unit operable to control the radiocommunication device; a timer operable to measure a standby time set bythe micro controller unit; a buffer circuit operable to receive anoutput signal of the timer; a logic circuit controlled by an outputsignal of the timer and an output signal of the micro controller unit;and a power-on switch controlled by the logic circuit in switching, andoperable to supply a power source to the micro controller unit andbuffer circuit. Further, the buffer circuit has no protection diodeconnected to an input terminal thereof on a power-source side. Besides,the micro controller unit is made to transition from an active statethereof to one of standby and power-source cutoff states during thestandby time, and restored from the one of the standby and power-sourcecutoff states to the active state according to an output from thebuffer. In the active state, the micro controller unit controls theradio communication device to transmit data captured by the sensor tothe base station.

[19] In the radio communication system as described in [18], the microcontroller unit decides to transition to the standby state or thepower-source cutoff state based on a length of the standby time set onthe timer.

[20] In the radio communication system as described in [18], a signalfor directing the micro controller unit to be restored to the activestate after the micro controller unit has transitioned to the standbystate and a signal for directing the micro controller unit to berestored to the active state after the micro controller unit hastransitioned to the power-source cutoff state are identical signals, andoutput by the timer.

2. Further Detailed Description of the Preferred Embodiments

Now, the embodiments will be described further in detail. The detaileddescriptions about forms for carrying out the invention will bepresented below based on the drawings. It is noted that as to all thedrawings to which reference is made in describing the forms for carryingout the invention, the members or parts having identical functions areidentified by the same reference numeral or character, and the repeateddescription thereof is omitted herein.

First Embodiment

The first embodiment of the invention will be described with referenceto FIGS. 1 to 9. FIG. 1 shows a configuration of a sensor net system,which is an example of the radio communication system in connection withthe invention. The sensor net system includes : sensor nodes (NOD) 100a, 100 b, 100 c, 100 d, 100 e, 100 f and so forth; base stations (BAS)101 a and 101 b; a network (NWK) 102; a server (SRV) 103; and a terminal(TRM) 105. The server 103 includes a database (DBS) 104. Incidentally,the subscripts a, b, c, and so forth represent that the members or partsreferred to by reference numerals accompanied by the subscripts areidentical to one another. Such subscripts are omitted in the descriptionbelow if not particularly required.

The sensor node 100 sends data acquired by use of the sensor to the basestation 101. The base station 101 manages a number of sensor nodes 100,and gathers data from the sensor nodes 100. The data so gathered aresent to the server 103 through the network 102 and then stored in thedatabase 104 inside the server 103. The server 103 analyzes the datastored in the database 104. The analyzed data and data stored in thedatabase 104 can be accessed from the terminal 105.

FIG. 2 shows an example of a sensor node for comparison with the sensornode in connection with the invention. The sensor node involved in thecomparative example includes: a micro controller unit (MCU) 200; a radiocommunication device (COM) 201; an antenna (ANT) 202; a timer (TIM) 203;a sensor (SENS) 204; a power source (PWR) 205; and a switch (SW) 206.MCU includes a central processing unit (CPU) 207; a volatile memory(RAM) 208; and a nonvolatile memory (ROM) 209. The radio communicationdevice 201 includes a transmitter (Tx) 210 and a receiver (Rx) 211.

The MCU 200 captures data from the sensor 204. The sensor 204 is used togauge its surrounding environment. Examples of the sensor 204 include atemperature sensor, a humidity sensor, an illumination sensor, anacceleration sensor, an infrared sensor and a barometric sensor. The MCU200 processes data captured from the sensor 204 on an as-needed basis,and sends out the data to the base station 101 through the transmitter210 in the radio communication device 201. The receiver 211 receivesdata from the base station 101. The data received from the base station101 are analyzed in the MCU 200, and are subjected to a processing suchas data retransmission on an as-needed basis.

As the power source 205 of the sensor node 100, a battery is often usedto eliminate the need for wiring. It is desired that the sensor node iscompact. Therefore, a battery which can be used therefor should be smallone, and it becomes necessary to reduce the power consumption of thesensor node. Examples of methods which have been conventionally used asmeans for reducing the power consumption include: a method by which e.g.in the case where the radio communication device 201 is not used, MCU200 turns off the switch 206, and cuts the power source of the radiocommunication device 201, thereby reducing the power consumption.

Known as another means for reducing the power consumption is a method bywhich MCU is made to transition to its standby mode in the case whereMCU processing is not required. After having set the time for which MCU200 is to remain on standby on the timer 203 in advance, MCU 200 is madeto transition to the standby mode, in which the power consumption iskept lowered. After elapse of the time set on the timer 203, the timer203 sends a control signal to MCU 200. Then, MCU 200 is restored to itsactive state from the standby state. After having been restored to theactive state, MCU 200 performs processes including data acquisition fromthe sensor 204, and communication with a base station 101 through theradio communication device 201. Now, it is noted that the standby staterefers to a state in which an unwanted clock inside MCU is stopped,whereby MUC power consumption is kept lowered. By exercising the controllike this, a conventional sensor node makes its power consumption lower.

However, the reduction of power consumption achieved by the above methodof reducing power consumption, namely the method of making MCU 200transition to the standby state, is sometimes insufficient.Specifically, e.g. an application software program such that a sensornode conducts various processes needs to use a high-performance MCU.With such high-performance MCU, even if MCU transitions to the standbystate, sometimes the effect of reduction of power consumption cannot beachieved sufficiently because of a large current leakage. In addition,the scaling down of semiconductor manufacturing processes is going aheadwith each passing year. For example, in the case of using MCU fabricatedby a manufacturing process designed for fine semiconductors, the amountof a leak current in the standby state, e.g. subthreshold leak current,is becoming unignorable. Therefore, in the invention, the sensor node100 is arranged so as to eliminate a leak current, and to achieve theeffect of reduction of power consumption sufficiently even in the caseof using a high-performance MCU or MCU manufactured by amicrofabrication process. FIG. 3 shows an example of the sensor node 100in connection with the invention. The sensor node 100 includes: a microcontroller unit(MCU) 200; a radio communication device (COM) 201; anantenna (ANT) 202; a timer (TIM) 203; a sensor (SENS) 204; a powersource (PWR) 205; a switch (SW) 206; a switch (SW) 301; a logic circuit(LGC) 300; and a buffer circuit (BUF) 302. MCU 200 includes: a centralprocessing unit (CPU) 207; a volatile memory (RAM) 208; and anonvolatile memory (ROM) 209. The radio communication device 201 has atransmitter (Tx) 210 and a receiver (Rx) 211.

The sensor node shown in FIG. 3 performs a sensing action andcommunication with the base station 101 like the sensor node involved inthe comparative example shown in FIG. 2. Specifically, MCU 200 processesdata captured by the sensor 204 into an appropriate format, theresultant data is sent to the base station 101 from the transmitter 210.Data from the base station 101 is received by the receiver 211 andprocessed by MCU 200. If the radio communication device 201 is not used,MCU 200 controls the switch 206 to cut the power supply to the radiocommunication device 201, thereby reducing the power consumption.

The sensor node shown in FIG. 3 differs from the sensor node of thecomparative example of FIG. 2 in that it has the logic circuit 300,switch 301 and buffer circuit 302. The switch 301 is placed in a powersupply line for MCU 200, and controlled by the logic circuit 300 inswitching. Signals from the MCU 200 and timer 203 are input to the logiccircuit 300. The buffer circuit 302 is located in a control signaltransmission line extending from the timer 203 to MCU 200. Now, theactions of the sensor node shown in FIG. 3 will be described withreference to FIGS. 4 to 8.

Now, a control circuit constituting a part of the sensor node 100 shownin FIG. 3 will be described in detail with reference to FIG. 4. As shownin the drawing, the control circuit includes the MCU 200, timer 203,logic circuit 300, switch 301 and buffer circuit 302, in which VDDdenotes a power-source line; VddM denotes a line for supply of powersource to MCU 200; Vtim denotes a control signal output from the timer203; Vint denotes a control signal supplied to MCU 200 from the buffercircuit 302 to which the control signal Vtim is input; and Vcnt denotesa control signal which MCU 200 inputs to the logic circuit 300.

The control signal Vcnt is activated in case that the power source VDDis cut by the switch 301. The control signal Vtim is activated at timewhen the timer 203 times out. The control signal Vint is one forordering an interruption used as a trigger to restore MCU 200 from thestandby state. This configuration enables not only transition of MCU 200to the standby state, but also cutting the power source of MCU 200.These actions will be described below.

FIG. 5 is an operation timing chart of the sensor node 100. As in thesensor node involved in the comparative example, MCU 200 can transitionto the standby state. Specifically, the sensor node works according tothe sequence as described below. MCU 200 sets the time for which MCU 200is to remain on standby on the timer 203, and then transitions to thestandby state. The timer 203 measures the standby time of MCU. After anelapse of the set standby time, the timer 203 causes the control signalVtim to fall down. The control signal Vtim is passed on to the buffercircuit 302, and then input, as the signal Vint, to MCU 200. Whendetecting the falling of the signal Vint, MCU 200 transitions from thestandby state to the active one.

In the case of cutting the power source of MCU 200, the control for thatis made as follows. MCU 200 sets, on the timer 203, the time for whichMCU 200 is to remain on standby. Then, MCU 200 changes the controlsignal Vcnt from High (Hi) level to Low (Lo) level. The signal Vcnt ispassed on to the logic circuit 300, and then the switch 301 is turnedoff. Thus, supply of the power source VddM to MCU 200 is cut. Also, thepower source of the buffer circuit 302 is connected together with thepower source VddM of MCU 200, and therefore the signal Vint, which is anoutput signal from the buffer circuit 302, is changed from High level toLow level in response of cutoff of the power source VDD by the switch301. In this way, the MCU 200 and buffer 302 are brought to apower-source cutoff state.

The sequence of restoration from the power-source cutoff state to theactive state is as follows. The timer 203 measures the standby time ofMCU. Then, after an elapse of the previously set standby time, the timer203 causes the control signal Vtim to fall down. The control signal Vtimis input to the logic circuit 300. Then, the logic circuit 300 turns onthe switch 301. As a result, the power source is supplied to MCU 200,and thus MCU 200 can be restored to the active state.

At the time of cutoff of power source of MCU 200, it is necessary tomake input and output pins of MCU 200 Low level. In other words, aprotection circuit is connected to input and output pins of MCUtypically. The protection circuit is intended for protecting an internalcircuit of MCU by means of forcing a current to pass through aprotection diode in the case where the voltage supplied to the input pinexceeds the voltage of a power source terminal. For example, diodes aredisposed e.g. between an input pin and power-source terminal and betweenthe input pin and power-source terminal so that they are connectedreversely to each other in direction in a typical power-supplyingcondition. Therefore, at the time of cutting the power source of MCU200, it is necessary to bring the input pin to Low level. To ensurethis, the buffer circuit 302 is arranged.

At the time of cutting the power source of MCU 200, the output signalVtim from the timer 203 is at High level. Therefore, if the signal linefor the signal Vtim is connected to MCU 200 directly, the voltage of theinput pin of MCU 200 exceeds the voltage of the power-source terminal ofMCU 200, a large current would flow into the protection circuit locatedin MCU 200, from the input pin toward the power-source terminal. On thisaccount, the buffer circuit 302 is placed between the timer 203 and MCU200, thereby to prevent a high-level voltage from being applied to theinput pin of MCU 200. The buffer circuit 302 has an input tolerantproperty. The input tolerant property refers to a trait such that alarge current never flows even if a voltage higher than the sourcevoltage is input. More specifically, the input tolerant property refersto a trait such that a large current never flows toward the power-sourceterminal even in the case where High level voltage is input to the inputpin in the condition where e.g. a structure which does not have diodesfor internal circuit protection between the input pin and power-sourcepin is embraced, and no source voltage is supplied to MCU. Even in thecase where the output from the timer 203 is at High level, if the powersource VddM is cut, the output Vint of the buffer circuit 302 is at Lowlevel because of using such buffer circuit 302. Therefore, during thetime when MCU 200 is in the power-source cutoff state, the input pinVint of MCU 200 can be made Low level.

In the case where no power source is supplied to MCU 200, the outputterminal of the MCU 200 is at Low level. Therefore, the control signalVcnt sent from MCU 200 to the logic circuit 300 has a polarity such thatthe control signal is at Low level during the time when the power sourceof MCU 200 remains cut. If the control signal Vcnt has a polarityopposite to this, the polarity of the control signal Vcnt will bechanged during the time when the power source remains cut, and thereforeit will be impossible to cut the power source properly.

Even in the case where the control signal Vtim from the timer 203 isreversed in polarity, if the buffer circuit 302 is not incorporated, thefollowing disadvantage will be brought about. First, considered is acase in which the control signal Vtim from the timer 203 stays at Lowlevel during the time when the power source remains cut, and the controlsignal Vtim is made to transition to High level after having measuredthe standby time. In this case, during the time when the control signalVtim stays at Low level, the input pin of MCU 200 is at Low level, and alarge current never flows into the input protection diode of MCU 200. Ifthe timer 203 turns the control signal Vtim to High level after havingmeasured the standby time, a voltage of High level will be supplied tothe input pin of MCU 200 before the power source is provided to MCU 200.Then, the voltage applied to the input pin of MCU 200 exceeds the sourcevoltage, and a large current will flow into the protection diode.Therefore, as described here, the buffer circuit 302 is required even ina case that the polarity of the control signal Vtim from the timer 203is reversed.

FIG. 6 shows relations of the polarity of the control signal Vtim sentfrom the timer 203 to MCU 200, the polarity of the control signal Vcntsent from the MCU 200 to the logic circuit 300, the state of the switch301, and the working state of MCU 200. In the case where both thesignals Vtim and Vcnt are at Low level, or the case where the signalVtim is at Low level and the signal Vcnt is at High level, the switch301 is turned on, whereby MCU 200 is put in action. Bringing the signalVcnt to Low level with the signal Vtim at High level turns off theswitch 301, and thus the power source for the MCU 200 is cut. To put MCU200 in standby state, the signal Vcnt is brought to High level to supplythe power source to the MCU 200. In addition, the output Vtim from thetimer 203 is made High level to measure the standby time.

FIG. 7 shows specific examples of the logic circuit 300 and switch 301.The logic circuit 300 includes an inverter 700 and an AND circuit(logical AND circuit) 701. The switch 301 includes a PMOS switch 702formed from a P-channel MOS transistor. This circuit structure enablesthe switch 301 to be controlled according to the truth values shown inFIG. 6. The logic circuit 300 needs to work during the time when thepower source of MCU 200 is cut. Therefore, connected with the logiccircuit 300 as the power source thereof is not the power source VddM,but the power source VDD, by which a constant power supply can beprovided.

FIG. 8 presents a diagram showing the transition of state of MCU 200. IfMCU 200 needs not work, MCU 200 is made to transition from the activestate 800 to the standby state 801. On receipt of the control signalVtim from the timer 203 serving as a trigger, MCU 200 transitions fromthe standby state 801 to the active state 800 through the restorationprocess 802. In the case of cutting the power source, MCU 200 is made totransition to the power-source cutoff state 803 by means of control ofthe control signal Vcnt. In response to the control signal Vtim from thetimer 203 serving as a trigger, the power source is supplied to MCU 200and as such, MCU 200 is made to transition to the active state 800 afterimplementation of the power-source recovery process 804.

As described above, it is possible to make MCU 200 transition to thestandby state 801 or power-source cutoff state 803 if MCU 200 is notused. Further, the trigger used at the time of restoration from thestandby state 801 to the active state 800, and the trigger used at thetime of restoration from the power-source cutoff state 803 to the activestate 800 are identical; the control signal Vtim serves as suchtriggers. Therefore, unlike the sensor node involved in the comparativeexample, in which MCU is made to transition only to the standby state,MCU can be made to transition to the power-source cutoff state as wellas the standby state in the sensor node in connection with theinvention.

With the sensor node in connection with the invention, the power sourceof MCU 200 can be cut. Therefore, it becomes possible to eliminate aleak current in connection with MCU 200. Also, it becomes possible tofurther reduce the power consumption when MCU 200 is out of use incomparison to the sensor node involved in the comparative example. Byarranging a buffer circuit having an input tolerant property between thetimer and MCU, it becomes possible to suppress the occurrence of thecurrent leakage thereby to reduce the power consumption by the sensornode during the time when MCU stays in the power-source cutoff state orthe time until the power supply is stabilized after the cancel of powercutoff. That is, while directly supplying an interrupt signal to aninterrupt terminal of a micro controller unit can cause a leak currentregardless of whether the interrupt signal is High-enable or Low-enableone, the occurrence of such leak current can be suppressed.

Further, by adoption of the sensor node in connection with theinvention, it becomes possible to selectively make MCU 200 transition tothe standby state 801, or to the power-source cutoff state 803 at thetime when MCU 200 is out of use. Thus, it becomes possible to select acondition which can further reduce the power consumption depending onthe standby time. The reason for this will be described with referenceto FIG. 9.

FIG. 9 is a graph of characteristic curves showing relations between thelife of a battery of the sensor node and the operation cycle. In thedrawing, an operation cycle (Tcyc) of the sensor node is taken for thehorizontal axis; a battery life (Tlife) is taken for the vertical axis.The battery life (Tlife) can be estimated based on the followingequation from a standby current (Is), an amount (Qa) of consumedelectric charge in action, an operation cycle (Tcyc), and a batterycapacity (Qb).

Tlife=Qb/(Is+Qa/Tcyc)   Eq. (1),

where the operation cycle Tcyc is assumed to be fixed. Making acomparison between the case where MCU 200 is made to transition to thestandby state 801, and the case where MCU 200 is made to transition tothe power-source cutoff state 803, the following are clear. That is, thestandby current Is is smaller in the transition to the power-sourcecutoff state 803 rather than the transition to the standby state 801.However, the amount Qa of consumed electric charge in action is smallerin the transition to the standby state 801 rather than the transition tothe power-source cutoff state 803. This is because the power-sourcerecovery process 804 takes a longer time than the restoration process802 from the standby state typically. Therefore, when the battery lifeTlife is estimated according to Eq. (1), results of estimation are asshown in FIG. 9. That is, with a short operation cycle Tcyc, the batterylife is longer in the case of transition to the standby state 801. Incontrast, with a long operation cycle Tcyc, the battery life is longerin the case of transition to the power-source cutoff state 803. This isbecause the amount Qa of consumed electric charge in action dominates inthe condition of a short operation cycle Tcyc, whereas the standbycurrent Is consumed on standby dominates in the condition of a longoperation cycle Tcyc.

Hence, by selecting whether to transition to the standby state 801 or tothe power-source cutoff state 803 according to the operation cycle, itbecomes possible to reduce the power consumption further, and thereforethe battery life Tlife can be made longer. If the battery life can beelongated, the frequency of battery replacements can be suppressed, andthe cost for the system maintenance can be reduced. Further, assumingbatteries identical in life, it becomes possible to use a smaller ormore compact battery, whereby the downsizing of the sensor node can beachieved.

While in the above description, a case in which the operation cycle Tcycis fixed has been explained, the operation cycle is not required to befixed. The operation cycle is synonymous with standby timesubstantially. Therefore, in the condition of a short standby time, thetransition to the standby state 801 shall be selected. In the conditionof a long standby time, the transition to the power-source cutoff state803 shall be selected. As a result, even when the operation cycle is notfixed, the power consumption can be reduced.

One of the advantages of the invention is that restorations of MCU fromthe standby state 801 and power-source cutoff state 803 to the activestate 800 can be performed by the same control signal Vtim. In otherwords, signals exactly identical to each other can be used as triggersignals for restorations from the standby state 801 and power-sourcecutoff state 803 to the active state 800. Therefore, the setting of thetimer 203 may be unchanged regardless of whether to make MCU transitionto the standby state or the power-source cutoff state.

Incidentally an example in which the control signal Vtim from the timer203 is used as a trigger to restore MCU to the active state 800 has beenexplained. However, the invention is not limited to the example. Forinstance, a system such that a signal from the sensor 204 is used as atrigger to restore can be arranged. Taking an example, an applicationsuch that when moving e.g. an object with a sensor node attachedthereto, an output from a vibration sensor of the sensor node is used asa trigger to restore MCU to its active state is conceived. In suchapplication, it is possible to perform sensing, data communication, andother actions. Using the control circuit of the invention in theapplication like this, if an object with a sensor node is movedfrequently, the reduction in power consumption is afforded by transitionto the standby state; if not frequently, the reduction in powerconsumption can be achieved by transition to the power-source cutoffstate. In other words, it is possible to achieve the reduction in powerconsumption by an optimum means according to a frequency with which anobject with a sensor node is moved.

FIG. 18 presents a flowchart showing an example of the flow of actionsof the sensor node. After start of working (1800), the sensor nodeperforms actions such as sensing of its surrounding environment, andcommunication with a base station (1801). Then, the sensor node decidesthe standby time until implementation of a subsequent action (1802). Thestandby time is decided based on sensed information, information gainedby communication or the like by a method which has been programmed onMCU 200 in advance. The standby time thus decided is set on the timer203 (1803). A judgment is made about whether the decided standby time islarger or smaller than a certain threshold (1804). In this judgment,whether or not the standby time is larger than the threshold is judgedby e.g. a conditional branch instruction. Then, which of the followingis to be performed is decided according to the result of the judgment:the process for cutting the power source; and the process for transitionto the standby state. The threshold is a time corresponding to theintersection of the power source cutting and the transition to standbyas shown in FIG. 9, which has been set on MCU 200 in advance. Otherwise,the threshold may be decided through communication with a base station.If the standby time thus decided is larger than the threshold, MCU 200selects the power source cutting (1805); if it is smaller than thethreshold, MCU 200 selects the transition to standby (1806). If thepower source of MCU 200 has been cut, the power source is recovered by asignal from the timer 203 after an elapse of a predetermined standbytime (1807). Similarly in the case of MCU staying on standby, MCU isrestored from the standby state by a signal from the timer 203 after anelapse of the predetermined standby time (1808). A series of the actionsis performed repeatedly.

By comparing the standby time with the threshold to select whether tocut the power source or to make MCU transition to the standby state inthis way, the means which can further reduce the power consumption canbe selected. Thus, it becomes possible to elongate the life of a batteryas far as possible.

Second Embodiment

Now, a sensor node in connection with the second embodiment of theinvention will be described with reference to FIGS 10 to 15. FIG. 10shows another example of the control circuit included in the sensor node100. The control circuit shown in the drawing includes: an MCU 200; atimer 203; a logic circuit 1000; a switch 301; a buffer circuit 302; andan activation circuit (WUC) 1001. In the drawing, VDD denotes apower-source line; VddM denotes a line for supply of power source to MCU200; Vtim denotes a control signal output from the timer 203; Vintdenotes a control signal input to MCU 200 after the signal Vtim hastraveled through the buffer circuit 302; Vcnt denotes a control signalwhich MCU 200 inputs to the logic circuit; and Vwuc denotes a controlsignal input to the logic circuit 1000 from the activation circuit 1001.

The control circuit shown in FIG. 10 is a circuit to ensure the supplyof power source to MCU 200 at power-on of the sensor node. It isnecessary to supply MCU 200 with the power source at power-on of thesensor node. In other words, the switch 301 must be turned on. Theswitch is controlled by the logic circuit 1000. Therefore, the controlto turn on the switch 301 at power-on is conducted at power-on bysupplying the output Vwuc from the activation circuit 1001 to the logiccircuit 1000. FIG. 11 presents a timing chart at power-on. FIG. 12 showscorrespondences between truth values of signals input to logic circuit1000 the and ON and OFF of the switch 301. The activation circuit 1001detects a leading edge of the power source VDD, keeps outputting at Lowlevel for a fixed length of time after the leading edge of the powersource VDD. As a result, the logic circuit 1000 controls the switch 301to ON. As the switch 301 is turned on, the power source VddM of MCU 200is set up, and provided to MCU 200. Thus, MCU 200 performs a resetprocess at power-on and then goes into the normal active state. Afterhaving entered the active state, MCU 200 controls the control signalVcnt to High level, whereby the switch 301 is turned on. After that, theoutput Vwuc of the activation circuit 1001 is changed to High level, andthe control circuit is put in a condition such that the output of thelogic circuit 1000 is controlled by the output Vcnt of MCU 200 and theoutput of the timer 203. The time during which the activation circuit1001 keeps its output signal Vwuc at Low level is set to be longer thana time during which the control signal Vcnt can be controlled after thecompletion of reset of MCU 200.

FIG. 13 shows specific examples of the logic circuit 1000 and switch301. The logic circuit 1000 includes a inverter 1300, and AND circuits1301 a and 1301 b. The switch 301 includes a PMOS switch 702. Suchcircuit configuration enables control of the switch 301 by use of thetruth values shown in FIG. 12. The logic circuit 1000 needs to be inaction at the time of cutting the power source of MCU 200, and thereforenot the power source VddM, but VDD is connected for the power source ofthe logic circuit.

Using the activation circuit 1001 in this way, it becomes possible toensure the power source of MCU 200 at power-on. Also, as in the case ofthe first embodiment, the standby state and the power-source cutoffstate can be switched to each other. FIG. 14 presents a timing chart atthe time of making MCU 200 transition to the standby state. MCU 200 setsthe time for which MCU 200 is to remain on standby on the timer 203 inadvance. Then, MCU 200 transitions to the standby state. After an elapseof the standby time set on the timer 203, the timer 203 causes thecontrol signal Vtim to fall down. The control signal Vtim is passed onto the buffer circuit 302, and then input, as the control signal Vint,to MCU 200. When detecting the falling of the control signal Vint, MCU200 transitions from the standby state to the active one.

FIG. 15 presents a timing chart at the time of cutting the power sourceof MCU 200. MCU 200 sets, on the timer 203, the time for which MCU 200is to remain on standby. After that, MCU 200 changes the control signalVcnt from High to Low level. The control signal Vcnt turns off theswitch 301 through the logic circuit 300. Thus, supply of the powersource VddM to MCU 200 is cut. The power source of the buffer circuit302 is connected with the power source VddM of MCU 200, and thereforethe control signal Vint, which is an output signal from the buffercircuit 302, is made to transition from High to Low level. After anelapse of the standby time, which has been set on the timer 203 inadvance, the timer 203 causes the control signal Vtim to fall down. Thecontrol signal Vtim is input to the logic circuit 300, and then thelogic circuit 300 turns on the switch 301. As a result, MCU 200 issupplied with the power source, whereby MCU 200 goes into the activestate.

As described above, also in the second embodiment, the buffer circuit302 ensures that the input pin of MCU 200 for accepting the controlsignal Vint is brought to Low level at the time of cutting the powersource of MCU 200. Thus, it becomes possible to selectively make MCU 200transition to the standby state, or to the power-source cutoff state.Besides, using the activation circuit 1001, it is possible to performcontrol so that MCU 200 is fed with the power source at power-on of thesensor node.

Third Embodiment

FIG. 16 shows another example of the control circuit included in thesensor node 100. The control circuit shown in FIG. 16 includes: an MCU200; a timer 203; a logic circuit 1600; a switch 301; a buffer circuit302; an activation circuit 1001; a regulator 1601; and a regulator 1602.In the drawing, VDD denotes a power-source line; VddM denotes aline forsupply of power source to the MCU 200 and buffer circuit 302; VddTdenotes a line for supply of power source to the timer 203, activationcircuit 1001 and logic circuit 1600; Vtim denotes a control signaloutput from the timer 203; Vint denotes a control signal input to MCU200 after the signal Vtim has traveled through the buffer circuit 302;Vcnt denotes a control signal which MCU 200 inputs to the logic circuit;and Vwuc denotes a control signal input to the logic circuit 1600 fromthe activation circuit 1001.

The control circuit shown in the drawing materializes a functioncomparable to that of the control circuit of FIG. 10, however it differsfrom the control circuit of FIG. 10 in that it has two types ofregulators 1601 and 1602. The regulator 1601 provides a power source tothe activation circuit 1001, timer 203 and logic circuit 1600. Theregulator 1602 supplies a power source to the MCU 200 and buffer circuit302. The logic circuit 1600 controls the regulator 1602 to turn ON/OFFthe power source VddM of the MCU 200 and buffer circuit 302. Theregulators are used for supplying a stable voltage. A stabilized powersource is necessary for e.g. an analog-to-digital converter operable toconvert analog data from a sensor into digital data. In addition, todraw out the best performance of the radio communication device,sometimes a stable power source is required. In the case of usingregulators as in the example of FIG. 16, directly controlling ON and OFFof the regulator 1602 from the logic circuit 1600 can eliminate the needfor a switch.

In communication between the timer 203 and MCU 200, it is preferablethat the power sources VddT and VddM are identical in voltage value.Hence, the two regulators 1601 and 1602, which output identical voltagesto each other, are used.

The regulator 1601 is used to operate only a device, such as the timer203, which consumes a small power, and therefore it may be a regulatorwhose output current is small. In general, a regulator small in outputcurrent consumes a small current. Therefore, to reduce the powerconsumption of the sensor node, the regulator 1601 which consumes asmall current is used. In contrast, the regulator 1602 serves to supplya power source to MCU 200 whose power consumption is relatively large.Therefore, a regulator which outputs a large current must be used as theregulator 1602. With such regulator which outputs a large current,current consumed by the regulator 1602 per se is relatively large.Hence, the regulator 1602 is also turned off at the time of cutting thepower source of MCU 200, thereby reducing the power consumption onstandby.

Using regulators as shown in the example of FIG. 16, it becomes possibleto supply a power source of a stable and fixed voltage. In addition, theneed for a switch is eliminated by controlling ON and OFF of theregulator 1602 from the logic circuit 1600. The regulator 1602 is turnedoff at the time of cutting the power source, which makes it possible toreduce the power consumption. Further, the regulator 1601 and regulator1602 are arranged to output identical voltages, whereby the voltages ofinput and output pins can be coincident with each other in communicationbetween the timer 302 and MCU 200. In this case, the current output bythe regulator 1601 maybe small, and therefore a regulator whose powerconsumption is small may be used.

Fourth Embodiment

FIG. 17 shows still another example of the control circuit included inthe sensor node 100. The control circuit shown in the drawing includes:an MCU 200; a timer 203; a logic circuit 1000; a buffer circuit 302; anactivation circuit 1001; a PMOS switch 1700; a switch 1701; and aresistor 1702. The features of the control circuit of FIG. 17 instructure are the switch 1701 and resistor 1702.

The switch 1701 is used only to turn on the PMOS switch 1700independently of an output from the logic circuit 1000. The output ofthe logic circuit 1000 depends on an output Vcnt from MCU 200.Therefore, in the case of supplying MCU 200 with a power source under acondition in which the output signal Vcnt of MCU 200 cannot becontrolled, the switch 1701 is required. Specific examples of such caseare the case of writing a program into an electrically-rewritablenonvolatile memory, e.g. a flash memory that MCU 200 has, and the caseof connecting an emulator with MCU 200 to debug a program. In suchcases, input and output pins of MCU 200 can be unstable, which can leadto instability of an output from the logic circuit 1000. On thisaccount, it is necessary to turn on the PMOS switch 1700 independentlyof an output from the logic circuit 1000. When using the switch 1701 tobring the gate voltage of the PMOS switch 1700 to GND level, the PMOSswitch 1700 can be turned on, whereby a source voltage can be providedto MCU 200. The resistor 1702 serves to prevent the control circuit frombeing directly connected to GND in the case of turning on the switch1701 under the condition where an output from the logic circuit 1000 isat High level. With the aid of the resistor 1702, the control of theswitch 1700 takes priority over the output by the logic circuit 1000,and therefore it becomes possible to control the PMOS switch 1700.

The switch 1701 like this is needed at times of downloading anddebugging a program, however it is not required in normal use.Therefore, it is desirable, from the viewpoint of downsizing, to mountthe switch 1701 on not a board for the main body of the sensor node,such as MCU 200, but a second board different from it. Specifically, aconnector 1704 is mounted on a board 1710 for the main body of thesensor node; a line connected with a gate of the PMOS switch 1700 and aline of GND are led out from the connector 1704. On the second board1720, the switch 1701 is mounted. The board 1710 for the main body ofthe sensor node and the board 1720 with the switch 1701 mounted thereonare connected with each other through the connector 1704 for debug.Thus, the main substrate on which the switch 1701 is not mounted can beused in normal use, and therefore a device can be scaled down. Inaddition, the number of parts on the main board for the sensor node canbe reduced, and therefore the const can be cut. For example, assuming anemulation, the board 1710 makes one on which the sensor node is actuallymounted, and the board 1720 serves as an emulation board. To theemulation board, an external terminal of MCU 200 is connected though aninterface cable (not shown).

Adopting the configuration as shown in FIG. 17, it becomes possible toprovide MCU 200 with a power source independently to an output from thelogic circuit 1000. Further, by mounting the switch 1701 on the secondboard other than the main board, the downsizing of devices and thereduction in cost owing to the decrease in the number of parts orcomponents are made possible.

Fifth Embodiment

FIG. 19 shows still another example of the control circuit included inthe sensor node 100. The control circuit shown in the drawing includes:an MCU 200; a sensor 1900; a sensor 1900; a logic circuit 1000; a switch301; a buffer circuit 302; and an activation circuit 1001. In thedrawing, VDD denotes a power-source line; VddM denotes a line for supplyof power source to the MCU 200 and buffer circuit 302; Vsens denotes acontrol signal output from the sensor 1900; Vint denotes a controlsignal input to MCU 200 after the signal Vsens has traveled through thebuffer circuit 302; Vcnt denotes a control signal which MCU 200 inputsto the logic circuit 1000; and Vwuc denotes a control signal input tothe logic circuit 1000 from the activation circuit 1001.

The control circuit of FIG. 19 uses the sensor 1900 instead of the timer203 used in the control circuit, which has been described above. As thesensor 1900, e.g. a vibration sensor, a temperature sensor, and aphotosensor may be used. Taking a vibration sensor as an example, thedescription will proceed here. In this case, an output from thevibration sensor is used as a trigger to restore MCU from thepower-source cutoff state or standby state to the active state.Conceived as an application thereof is that MCU is restored to theactive state e.g. in the case that an object with the sensor nodeattached thereto is moved, and then the sensing or data communicationcan be performed. Using the control circuit of FIG. 19 in theapplication like this, if an object with a sensor node is movedfrequently, the reduction in power consumption is afforded by transitionto the standby state; if not frequently, the reduction in powerconsumption can be achieved by transition to the power-source cutoffstate. In other words, it is possible to achieve the reduction in powerconsumption by an optimum means according to a frequency with which anobject with a sensor node is moved.

Further, if using a temperature sensor, for instance, the sensor nodecan be made to work only at the time when the temperature changes.Otherwise, using a photosensor, an application such that the sensor nodeis made to work only when the brightness changes is possible.

While the invention made by the inventor has been specifically describedabove based on the embodiments thereof, the invention is not limited tothe embodiments. It will be obvious that various changes andmodifications may be made without departing from the scope hereof.

1. An electronic circuit comprising: a micro controller unit; a timeroperable to measure a standby time under control of the micro controllerunit; a buffer circuit operable to receive an output signal of thetimer; a logic circuit controlled by the output signal of the timer andan output signal of the micro controller unit; and a power-on switchcontrolled by the logic circuit in switching, and operable to supply apower source to the micro controller unit and buffer circuit, whereinthe buffer circuit has no protection diode connected with an inputterminal thereof on a power-source side, and the micro controller unitis made to transition from an active state thereof to one of standby andpower-source cutoff states during the standby time, and restored fromthe one of the standby and power-source cutoff states to the activestate according to an output from the buffer.
 2. The electronic circuitaccording to claim 1, wherein the micro controller unit decides totransition to the standby state or the power-source cutoff state basedon a length of the standby time set on the timer.
 3. The electroniccircuit according to claim 1, wherein a signal for directing the microcontroller unit to be restored to the active state after the microcontroller unit has transitioned to the standby state, and a signal fordirecting the micro controller unit to be restored to the active stateafter the micro controller unit has transitioned to the power-sourcecutoff state are identical signals, and output by the timer.
 4. Theelectronic circuit according to claim 1, wherein the logic circuitswitches the output signal of the micro controller unit from High to Lowlevel, thereby to turn off the power-on switch, and to cut a powersource of the micro controller unit and buffer circuit, and the logiccircuit turns on the power-on switch to power on the micro controllerunit and buffer circuit in response to changeover of the output signalof the timer from High to Low level.
 5. The electronic circuit accordingto claim 4, wherein after having transitioned from the active state tothe standby state, the micro controller unit is restored to the activestate in response to changeover of the output signal of the timer fromHigh to Low level.
 6. The electronic circuit according to claim 1,wherein the micro controller unit inputs, through its interruptterminal, the output signal of the buffer for restoration from thestandby state to the active state.
 7. The electronic circuit accordingto claim 1, wherein the buffer circuit has a protection diode betweenthe input terminal and a ground terminal.
 8. The electronic circuitaccording to claim 1, further comprising an activation circuit connectedwith the logic circuit, wherein the activation circuit remains waitingfor a predetermined length of time after its power-on, and then changesits output, and the logic circuit keeps the power-on switch ON stateuntil the activation circuit changes its output regardless of states ofoutputs from the micro controller and the timer.
 9. The electroniccircuit according to claim 1, further comprising a connector for debug,which enables supply of a signal for forcing the power-on switch toshift into ON state.
 10. An electronic circuit, comprising: a microcontroller unit; a timing-generation sensor operable to output a certaindetection result; a buffer circuit which accepts an output of thetiming-generation sensor; a logic circuit controlled by the outputsignal of the timing-generation sensor and an output of the microcontroller unit; and a power-on switch controlled by the logic circuitin switching, and operable to supply a power source to the microcontroller unit and buffer circuit, wherein the buffer circuit has noprotection diode connected to an input terminal thereof on apower-source side, and the micro controller unit is made to transitionfrom an active state thereof to one of standby and power-source cutoffstates by the time that the timing-generation sensor outputs the certaindetection result, and restored from the one of the standby andpower-source cutoff states to the active state according to an outputfrom the buffer.
 11. An electronic circuit, comprising: a microcontroller unit; a timer operable to measure a standby time undercontrol of the micro controller unit; a buffer circuit operable toreceive an output signal of the timer; a logic circuit controlled by theoutput signal of the timer and an output signal of the micro controllerunit; a first regulator controlled by the logic circuit in itsactivation, and operable to supply a power source to the microcontroller unit and buffer circuit; and a second regulator operable tosupply a power source to the timer and logic circuit, wherein the buffercircuit has no protection diode connected to an input terminal thereofon a power-source side, and the micro controller unit is made totransition from an active state thereof to one of standby andpower-source cutoff states during the standby time, and restored fromthe one of the standby and power-source cutoff states to the activestate according to an output from the buffer.
 12. The electronic circuitaccording to claim 1, further comprising: a sensor; a radiocommunication unit; and a power-on switch for radio communication,operable to control a power source of the radio communication unit,wherein the power-on switch for radio communication is controlled by themicro controller unit, and the radio communication unit transmits datafrom the sensor under control of the micro controller unit.
 13. A radiocommunication system, comprising: a plurality of radio communicationdevices; and a host device which communicates with and manages theplurality of radio communication devices, wherein at least one of theplurality of radio communication devices is the electronic circuitaccording to claim
 12. 14. The electronic circuit according to claim 10,further comprising: a sensor; a radio communication unit; and a power-onswitch for radio communication, operable to control a power source ofthe radio communication unit, wherein the power-on switch for radiocommunication is controlled by the micro controller unit, and the radiocommunication unit transmits data from the sensor under control of themicro controller unit.
 15. A radio communication system, comprising: aplurality of radio communication devices; and a host device whichcommunicates with and manages the plurality of radio communicationdevices, wherein at least one of the plurality of radio communicationdevices is the electronic circuit according to claim
 14. 16. Theelectronic circuit according to claim 11, further comprising: a sensor;a radio communication unit; and a power-on switch for radiocommunication, operable to control a power source of the radiocommunication unit, wherein the power-on switch for radio communicationis controlled by the micro controller unit, and the radio communicationunit transmits data from the sensor under control of the microcontroller unit.
 17. A radio communication system, comprising: aplurality of radio communication devices; and a host device whichcommunicates with and manages the plurality of radio communicationdevices, wherein at least one of the plurality of radio communicationdevices is the electronic circuit according to claim
 16. 18. A radiocommunication system, comprising: a radio communication terminal; a basestation operable to communicate with the radio communication terminal byradio; a server operable to process data passed from the radiocommunication terminal through the base station, wherein the radiocommunication terminal includes a sensor operable to capture data, aradio communication device operable to transmit data captured by thesensor to the base station, a micro controller unit operable to controlthe radio communication device, a timer operable to measure a standbytime set by the micro controller unit, a buffer circuit operable toreceive an output signal of the timer, a logic circuit controlled by anoutput signal of the timer and an output signal of the micro controllerunit, and a power-on switch controlled by the logic circuit inswitching, and operable to supply a power source to the micro controllerunit and buffer circuit, the buffer circuit has no protection diodeconnected to an input terminal thereof on a power-source side, the microcontroller unit is made to transition from an active state thereof toone of standby and power-source cutoff states during the standby time,and restored from the one of the standby and power-source cutoff statesto the active state according to an output from the buffer, and themicro controller unit controls the radio communication device totransmit data captured by the sensor to the base station.
 19. The radiocommunication system according to claim 18, wherein the micro controllerunit decides to transition to the standby state or the power-sourcecutoff state based on a length of the standby time set on the timer. 20.The radio communication system according to claim 18, wherein a signalfor directing the micro controller unit to be restored to the activestate after the micro controller unit has transitioned to the standbystate and a signal for directing the micro controller unit to berestored to the active state after the micro controller unit hastransitioned to the power-source cutoff state are identical signals, andoutput by the timer.